1. Field of the Invention
This invention relates to an image forming apparatus such as a printer or copying machine involving a method in which a toner image formed on a photosensitive body is first transferred onto an intermediate transfer body, and the first transferred toner image on the intermediate transfer body is transferred and fixed on a recording paper.
2. Description of the Related Art
Conventional image forming apparatuses used widely utilizing the conventional image forming technique, for example, in which an electrostatic latent image is formed on a photosensitive body, the latent image is developed with dry toner to form a toner image, the toner image is transferred and fixed electrostatically onto a recording medium, are disadvantageous in that uneven photographic density is caused, or powder toner is scattered in a transfer area, and an image is poor in resolution and dot reproducibility.
These disadvantages are largely due to the transfer process for transferring a toner image formed on a photosensitive body onto a recording medium electrostatically. In detail, in the transfer method utilizing an electrostatic transfer system, it is difficult to transfer a toner image formed on a photosensitive body uniformly and efficiently. The toner transfer efficiency increases with increasing electric field (referred to as Et hereinafter) applied to the toner layer, however, the transfer efficiency and Et turn to decrease at the certain electric field due to Paschen discharge. In other words, the transfer efficiency has a peak at a certain Et value. The peak value of the transfer efficiency is not 100% but is at most 95%. Because the transfer efficiency of a toner layer depends on Et, unless Et is maintained constant independently on the evenness of a toner layer and a recording medium typically such as a paper, and evenness electrical property, the resultant transfer efficiency becomes different depending on the thickness of the toner layer and transfer position of the recording medium. In the case that a toner image formed on a recording medium is monochrome and has a thin layer thickness, the uneven recording medium and uneven electrical property cause an uneven image. The same is true for transfer process in which a plurality of monochrome toner images formed independently on photosensitive bodies are transferred onto a recording medium one on another, an uneven recording medium and uneven electrical property result in an uneven image. In other words, though the difference between a portion where a plurality of toner images are transferred one on another and a portion where a monochrome toner image is transferred but multi-colors are not transferred can be suppressed electrostatically, the unevenness of a recording medium and unevenness of electrical property cannot be compensated.
On the other hand, in transfer using the intermediate transfer body in which monochrome color images formed independently on photosensitive bodies are transferred onto an intermediate transfer body having no unevenness and controlled property one on another, an even image is obtained on the intermediate transfer body. The toner image on the intermediate transfer body is multi-layered, and composed of maximum three-layers and minimum one or less layer depending on the portion. It is difficult to apply a constant electric field to transfer electrostatically these toner layers simultaneously and unevenly on recording paper which is typical of the recording medium, and Et is uneven. As the result, in electrostatic image transfer process, not all the color image formed on an intermediate transfer body formed one on another is transferred onto a recording medium, some of the color image remains on the intermediate transfer body. The remaining toner image depends on the toner thickness of the color image formed on the intermediate transfer body. Therefore color balance of the color image obtained on a recording medium is deviated, and the desired color image cannot be obtained. Further, because of irregularity on the surface of the paper namely recording medium, the paper fits on the intermediate transfer body imperfectly to cause uneven gaps, and a transfer electric field is disturbed and toner is scattered due to mutual Coulomb repulsion of powder toner, thus the image quality becomes poor.
To cope with this problem, Japanese Published Examined Patent Application No. Sho 46-41679 discloses a method in which a toner image formed on a photosensitive body is adhered and transferred on an intermediate transfer body consisting of elastic material, and then the toner image is melted by heating and transferred from the intermediate transfer body onto a recording medium. Because a toner image is transferred from an intermediate transfer body onto a recording medium non-electrostatically in this method, the image quality is unlikely to deteriorate so significantly during transfer process as described herein above.
Further, for example, Japanese Published Unexamined Patent Application No. Hei 2-108072 discloses a method in which toner images of different colors are transferred electrostatically on an intermediate transfer body one on another, the multi-color multi-layer toner image is melted on the intermediate transfer body, and then the melted multi-layer toner image is transferred onto a recording medium to obtain a color copy. Because a toner image is transferred non-electrostatically onto a recording medium in this method as well, the image quality is unlikely to deteriorate so significantly. This method is called a transfer fixing method.
Concerning to the image forming apparatus which utilizes this transfer fixing method, U.S. Pat. No. 2,990,278 specification, Japanese Published Unexamined Patent Application No. Hei 5-19642, and Japanese Published Unexamined Patent Application No. Hei 5-249798 disclose an apparatus for transferring in which an intermediate transfer body and a recording medium are thermally brought into tight contact with each other and are pressed in order to transfer completely the toner image from the intermediate transfer body onto the recording medium, then cooled until cohesion between toner particles becomes stronger than the adhesion between toner and the intermediate transfer body, thereafter the recording medium is separated from the intermediate transfer body. According to this method, the high quality image of high toner transfer, excellent color balance and high gloss, and excellent toner transparency is obtained. However, in transfer fixing of toner using this intermediate transfer body, color copying toner currently used adheres on the surface of a member used for heating and melting, that is, so-called offset phenomenon occurs, unless releasing agent such as silicone oil is used.
On the other hand, to utilize effectively the advantage of the intermediate transfer body, various studies have been conducted in relation to the structure of the uppermost layer of the intermediate transfer body.
For example, silicone rubber, fluorine-contained resin, and fluorine-contained rubber which contains fluorine-contained resin dispersed therein are known as heat resistant and toner releasing materials.
Because the intermediate transfer body surface is in contact with the photosensitive body surface in an apparatus which utilizes an intermediate transfer body, a releasing agent such as silicone oil cannot be supplied like the conventional fixing unit. To solve this problem, a method in which a recording medium is separated from an intermediate transfer body after toner is cooled to a temperature lower than the melting point of the toner has been proposed. This method is disclosed in the patents described herein above.
However, because fluorine-contained resin and fluorine-contained rubber which contains fluorine-contained resin dispersed therein used as a surface material of an intermediate transfer body is less adhesive between an intermediate transfer body and a recording medium, toner is separated before the toner is cooled sufficiently to cause offset and uneven gloss. A tight contact means is required to prevent this problem, thus the requirement leads to a complex and large sized apparatus. Because fluorine-contained resin and fluorine-contained rubber which contains fluorine-contained resin dispersed therein are inherently poor in toner releasing property and have hard surface in comparison with silicone rubber, the surface of these materials does not fit to the toner image surface, and a low height toner image located near a high height toner image is not transferred because of large difference in height to cause transfer lacking. Further the difference in toner image height causes the gloss difference to result in uneven gloss. In the case of an image in which fine lines are located closely each other, fire lines are diffused and joined together at some portions. As described herein above, the above-mentioned materials result in poor image quality in comparison with silicone rubber particularly in the case that multi-colors are transferred and fixed as in the case of color image copying.
Therefore, silicone rubber is popularly used as intermediate transfer body surface material for the process in which a color image is transferred from the photosensitive surface onto an intermediate transfer body, and the transferred image is transferred and fixed on a recording paper, because silicone rubber results in good image quality.
However, even though silicone rubber is used as surface material of an intermediate transfer body, a process in which a recording paper is fit to a toner image formed on the intermediate transfer body surface, and then the recording paper is separated from the intermediate transfer body after cooling to a temperature lower than the melting point of the toner should be operated.
While separation after cooling of melted toner results in no offset onto the intermediate transfer body surface, the surface of the toner image on the recording medium separated from the intermediate transfer body surface is a replica of the surface of the intermediate transfer body. In other words, just as making a mold, the surface configuration of the intermediate transfer body is copied on the surface of the toner image, a smooth intermediate transfer body surface results in a high gloss toner image, on the other hand, a roughed cloudy intermediate transfer body surface results in a low gloss toner image. This is the feature of this method.
There are many independent toners called as fog toner other than toner image for forming an image on the photosensitive body surface. Silicone rubber transfers also mostly fog toner on the photosensitive body onto the intermediate transfer body surface because of elasticity and adhesive property of silicone rubber. In the conventional case that a toner image on the photosensitive body surface is transferred directly onto a recording paper utilizing electrostatic force, fog toner is selectively transferred utilizing electrostatic force, and event though it is transferred onto a recording paper, fog toner is unrecognizable.
When an intermediate transfer body having a silicon rubber coating is used, fog toner is mostly transferred and fixed on a recording medium to result in the poor image quality. Such poor image quality is a problem.
Because of excellent leveling property of silicone rubber, silicone rubber coated on an intermediate transfer body surface forms a very smooth surface. The friction efficiency between silicone rubber and the smooth surface of a photosensitive body is very high. For forming a color image namely copying or printing, a color is synthesized by using three or more color toner images formed one on another. In this case, the registration of each color, namely position deviation, affects strongly the image quality. The smooth intermediate transfer body surface gives a high friction coefficient between a photosensitive body, and causes slipping between a driving roller for driving the intermediate transfer body and the back side of the intermediate transfer body. High friction coefficient between the driving roller and the back side of the intermediate transfer body for increasing the driving force often causes waving of the intermediate transfer body due to mutual tension between intermediate transfer body and photosensitive body to result in non-flat surface of the intermediate transfer body because of inconsistency between the direction of a force exerted from the driving roller and the direction of a force exerted from the photosensitive body due to poor mechanical accuracy though it causes no problem if both force directions are coincident. As the result, the toner image on the photosensitive body is not transferred as it is to cause a defective image. Such defective image is a problem.
To reduce the friction coefficient of rubber, method in which the surface is roughed is used. Some methods have been known. One of these methods is a spray coating method in which spray condition for spraying silicone rubber is selected so that atomization of silicone rubber is unlikely to occur by changing, for example, coating condition such as temperature, humidity, and spray distance, and viscosity of silicone rubber, and then a rough surface is obtained. By applying this method, micro-waving surface is obtained but the wave surface is smooth, the resultant friction coefficient is still large. Blasting treatment, in which sands or steel particles are blasted onto a surface, roughs the entire surface so as to make recesses on the rubber surface, the friction coefficient decreases only slightly. The fog toner transfer is not reduced. The image gloss is reduced significantly, and the surface is difficult to rough evenly to cause unevenness, and the image quality becomes poor. Other coating methods such as blade coating and dip coating also result in long swell with smooth mirror-like surface, the problem which the inventors of this invention addressed on is not solved.
Japanese Published Unexamined Patent Application No. Sho 59-50473 mentions the surface roughness of the intermediate transfer body and discloses a method for controlling the surface roughness by spray coating, however, the object of this invention is to improve the life of rubber, so the friction coefficient between an intermediate transfer body and photosensitive body cannot be reduced so significantly as the intermediate transfer body is slipped on the photosensitive body and is controlled, or fog toner transfer cannot be reduced significantly to an unrecognizable level. Japanese Published Unexamined Patent Application No. Hei 5-333711 discloses a method for prescribing the roughness of the intermediate transfer body surface to prevent transferring lacking, however, the surface roughness described in this disclosure is insufficient for obtaining the intermediate transfer body having the silicone rubber surface which satisfies high gloss requirement desirable as the color image and excellent intermediate transfer body driving running controllability requirement for preventing image deviation, namely requirement for low friction coefficient between the intermediate transfer body and photosensitive body and reduced fog toner transfer.
In view of the above-mentioned disadvantage, the inventors of the present invention proposes an intermediate transfer body having the peak area and smooth recess area which is used in an image forming apparatus for transferring and fixing a toner image from a photosensitive body onto a recording medium using the intermediate transfer body having the silicone rubber surface wherein the image gloss is as high as desirable for color image, the friction between the intermediate transfer body and photosensitive body is reduced so that intermediate transfer body driving running control becomes easy, and the fog toner transfer is reduced to prevent the deterioration of image quality, and an image forming apparatus which uses the intermediate transfer body.
However, the image gloss preference is different individually, most prefer high gloss (70% or higher) but some performs medium gloss (40 to 70%), which is currently used color copy machines reproduce usually.
To achieve such medium gloss using an intermediate transfer body having the peak area and smooth recess area proposed by the inventors of the present invention, the peak area is increased, however, an experiment reveals a problem of poor intermediate transfer body driving running performance due to an increased friction coefficient between the intermediate transfer body and the photosensitive body with an increasing contact area between the intermediate transfer body and the photosensitive body.
On the other hand, it is possible to make the intermediate transfer body driving running control easy by reducing the friction coefficient between the intermediate transfer body and the photosensitive body, however, the reduced friction coefficient results in low image gloss of about 20%.